Dorkbot-variant infection is unusual because cyber criminals exploited a flaw in the file-sharing site MediaFire to spread the malware

A new variant of the Dorkbot malware was discovered spreading through Facebook's internal chat service, in an attempt by cyber criminals to harvest users' passwords.

Bitdefender researchers discovered links to the malware circulating among Facebook users in the United States, India, Portugal, the United Kingdom, Germany, Turkey and Romania. The security firm did not know the extent of the infection on Monday, but said it counted 9,000 malicious links to the malware in 24 hours.

Bitdefender notified Facebook, which cleared the site of the infection on Tuesday.

"They' were very prompt in their response and they probably protected a lot of users," Bogdan Botezatu, senior e-threat analyst for Bitdefender said.

The infection was unusual because the criminals exploited a flaw in the file-sharing site MediaFire to spread the malware. From infected computers, the malware sent links to the malicious app to the victim's Facebook friends via the chat service.

The link had a JPG extension to trick the recipient into believing it led to an image file. Clicking on the link would download and run the malware.

Once in the computer, the application would monitor a victim's browsing activity and harvest passwords from the browser, Botezatu said. Infected computers could also be used in distributed denial of service (DDoS) attacks.

Dorkbot is capable of updating itself and can prevent antivirus software from running security updates. It can also download software from a command-and-control (C&C) server for distributing spam and ransomware.

Dorkbot and its variants have been circulating the Web since 2003. Before the latest variant, the malware was last seen in the wild in 2011, Bitdefender says. At that time, it spread through several instant-messaging clients, including Yahoo Messenger, Pidgin and Xchat.

Dorkbot is spread through a botnet that uses the Internet Relay Chat (IRC) protocol to spread.

"Dorkbot's primary purpose is to infect as many computers as possible," Botezatu said. Besides building a large a botnet, the malware's primary purpose is credential harvesting and DDoS attacks.

In January, the spam-fighting group Spamhaus took down many of the domain names of another IRC-based botnet that spread the Virut malware. Once installed in a computer, Virut connected to an IRC server using an encrypted connection, allowing attackers to control the infected computer.

IRC bots go back to 1993 with the benevolent EggDrop bot. One of the first malicious botnets was GTbot discovered in 1998.

cyber criminals will use a centralized command-and-control structure based on IRC, partly because of the extensive knowledge and code base around the technology. In 2000, criminals also started using the decentralized peer-to-peer technology, which makes the botnet much harder to shutdown.